Imaging systems such as printers, fax machines, and copiers are virtually omnipresent, and can be found in homes and offices worldwide. The development of such systems has facilitated improvements in communication that have in turn fostered a sea of change in the way people live and work. Telecommuting, paperless offices, and intra-office networks represent but a few examples of the advancements that have been made possible by modern imaging systems.
Since these systems have become crucial to everyday existence, their reliability and smooth operation is paramount. It is therefore vitally important to design imaging systems so that downtime and work interruptions are minimized. This can be a daunting challenge, given the relative complexity of systems in which sheet material must be infed, moved through the imaging process, and outfed in a matter of seconds.
It has been found that the difference in weight between smaller and larger sheet sizes, differences in weight between thicker and thinner sheets, and different sheet surface textures can present problems in sheet feeding throughout the imaging system. For each combination of these factors, successful transportation of sheet material depends upon applying the proper amount of applied force on the sheet with a feed mechanism such as a roller. The combination of forces is shown in FIG. 1. When torque T is applied to the roller R, the combination of torque and normal force N produces a transport force P, which causes the sheet material to move. Accurate application of the applied force A transmitted through the roller R allows the system designer to produce the desired normal and transport forces appropriate for a particular sheet material.
The consequences of incorrect forces can be problematic. Using the infeed mechanism as an example, if the applied force is too low, sheets can have "no-pick" problems, where the transport force is insufficient to remove the sheets from the stack. At the other end of the spectrum, if the force is too great, the result may be "multi-feed" problems, wherein the transport force introduces several sheets into the feed mechanism simultaneously. Excess force can also cause deformation of one or more of the underlying sheets.
Known approaches to address these difficulties involve using springs or weights to produce a load on the feed roller. Spring-loaded systems typically use coil or leaf springs to apply force to the feed rollers. Examples of such systems are set forth in U.S. Pat. Nos. 5,163,666 to Kuo and 5,474,288 to Lo et al.
Spring systems are effective to a degree, but have significant drawbacks as well. For example, spring constants often have relatively high tolerances about their stated nominal values, sometimes in the range of 10% to 20%. Such variation in force produces unacceptable variation in transport force in an imaging system. Further, if the feed roller must accommodate different stack heights, the applied force exerted by the spring frequently changes the load on the sheet material as the angle and height of the roller varies.
Systems using weights employ a fixed, known mass to apply force to the feed roller. Such systems are relatively accurate for applying a specified load. Unfortunately, the required forces for feed rollers often dictate a weight whose physical size would be impractical to incorporate into the housing design for an imaging system. Additionally, the effect of the weight on the roller would be dependent on the orientation of the system, thus requiring a level resting surface. Further, weight systems can be subject to the same stack height problems discussed with reference to spring systems.
Neither spring systems nor weight systems has the ability to compensate for other variables in the imaging system. These variables can include such things as changes in friction at hinge points of arms and the like due to manufacturing tolerances, temperature variations, or wear. These have the potential to change the applied force, thus negatively affecting the transport force.
It can thus be seen that the need exists for a reliable and predictable way to transmit applied force to a feed roller associated with imaging devices such as printers, copiers, and fax machines.